FAA Airline Transport Pilot (ATM)Navigation, Air Traffic Control and ProceduresHard
An aircraft is flying directly over a VOR/DME station at 6,000 feet MSL, and the station's elevation is at sea level. Approximately what slant-range distance will the DME display at that moment?
- A0.0 NM
- B10.0 NM
- C1.0 NM
- D6.0 NM
Show answer & explanationAnswer & explanation
Correct answer: C. 1.0 NM
DME measures line-of-sight slant range, not ground distance. When directly overhead a station at 6,000 feet MSL, the slant range approximately equals the aircraft's altitude converted to nautical miles: 6,000 ft ÷ 6,076 ft/NM ≈ 1.0 NM, even though ground distance is zero.
Why the other options are wrong
- A. DME cannot read zero when the aircraft is above the station due to slant-range geometry.
- B. This distance would require either much greater altitude or ground distance from the station.
- D. This would only occur at very low altitude with an offset ground distance of nearly 6 NM.
DME Slant Range Error
DME measures the direct line-of-sight (slant) distance between the aircraft and the ground station, which differs from actual horizontal ground distance, especially when close to or overhead the station.
- Slant range = line-of-sight distance, not ground distance
- Error is greatest when close to the station or at high altitude
- Error becomes negligible at long distances from the station
- Directly overhead, DME reads approximately altitude in NM (using 6,076 ft/NM)
Memory trick: 'DME sees a straight line, not the ground line — closer in, bigger the error.'